With an increase in the amount of transmission information, optical circuits have been used in conjunction with electric circuits in recent electronic devices and the like. As an example, an opto-electric hybrid board R0 in which an electric circuit board E and an optical waveguide (optical circuit) W are stacked together, as shown in FIG. 9, has been proposed (see PTL 1, for example). The electric circuit board E includes an insulative substrate 51 and electric circuits 52 formed on the back surface of the insulative substrate 51. The optical waveguide W is formed on the front surface of the insulative substrate 51, and includes an under cladding layer 54 formed on the front surface of the insulative substrate 51, cores (optical path) 55 patterned on the front surface of the under cladding layer 54, and an over cladding layer 56 formed on the front surface of the under cladding layer 54 so as to cover the cores 55.
In general, the opto-electric hybrid board R0 is produced through a roll-to-roll step. Specifically, the production of the opto-electric hybrid board R0 is as follows. First, a roll obtained by winding a strip-shaped electric circuit board sheet in a roll form is prepared. The strip-shaped electric circuit board sheet includes a plurality of electric circuit boards E disposed at predetermined spacings. In the strip-shaped electric circuit board sheet, the electric circuits 52 disposed at predetermined spacings are formed on the back surface of an insulative strip-shaped sheet, and portions of the insulative strip-shaped sheet which correspond to the electric circuits 52 serve as the insulative substrate 51. While the electric circuit board sheet is unwound from the roll, the under cladding layer 54 is formed on the entire front surface (a surface on the opposite side from the surface on which the electric circuits 52 are formed) of the electric circuit board sheet by a photolithographic process. Subsequently, the cores 55 and the over cladding layer 56 are patterned in the order named on the front surface of the under cladding layer 54 by a photolithographic process. In this manner, a plurality of optical waveguides W are formed on the front surface of the strip-shaped electric circuit board sheet, so that a strip-shaped product cluster sheet is produced. Then, the strip-shaped product cluster sheet is passed through rollers and wound in a roll form. Thus, a product cluster roll is provided. Thereafter, the product cluster sheet is unwound from the product cluster roll and cut into opto-electric hybrid board sheets (with reference to FIG. 10) each having a predetermined number of opto-electric hybrid boards R0. In general, such an opto-electric hybrid board sheet is sized to have an area that can be exposed to light at a time during the formation of the optical waveguides W by a photolithographic process. Then, the opto-electric hybrid boards R0 are cut from the opto-electric hybrid board sheet.
In the opto-electric hybrid board sheet, the under cladding layer 54 is formed on the entire front surface, as shown in FIG. 10. The under cladding layer 54 is shown as shaded with broken diagonal lines in FIG. 10 for ease of understanding of the state of formation of the under cladding layer 54. The cores 55 and the over cladding layer 56 are not shown in FIG. 10.